The present application relates to milling cutters. More particularly, there is provided an end mill configured to substantially reduce or eliminate vibrations of the cutter itself and of the work piece.
The milling process is by its very nature a non-continuous form of machining. A wide range of cutters are available “off the shelf,” and it is up to the user to select the type required. The cutter used may have up to about 20-30 teeth, depending primarily upon the diameter of the cutter and its type, and on further factors, such as the material of which the cutter is constructed, the material of the work piece, whether the cutting operation is for finishing or roughing, the required or desired cutter life, and the like.
As can be expected from any non-continuous type of machining, vibrations are generated by milling, and such vibrations may range in amplitude from negligible to severe. During machining with an end mill, the tool is generally subject to both bending and torsional forces; these being of an intermittent nature due to a tooth contacting or ceasing to contact the face being machined. As those visiting a working machine shop are aware, these vibrations generate sounds which are in frequencies and amplitudes to which the human ear is sensitive.
The work piece being machined is also set into vibration, the nature of which will be significant for a large hollow item and will be of no consequence for a solid well-supported and securely clamped work piece.
Noticeable vibration, sometimes referred to as chattering, is detrimental to machining not only because of the generated noise. Such vibrations are directly responsible for a poor surface finish on the work piece, as well as for a shortening of the life of the cutter and reduced accuracy in machining.
Undesired vibrations may be present in the cutting tool, or in the work piece, and can be present in both.
A severe problem arises when the frequency of vibration of the cutter corresponds or is proximate to the natural frequency of the work piece, causing resonance. The resulting greatly increased amplitude makes it impossible to produce acceptable work and the generated sound can be most disturbing. Breakage of a cutter such as an end mill or spoilage of the work piece is also likely. The use of a more rigid cutter and the application of additional work piece supports would increase vibration frequency to a safe and non-audible level and greatly reduce amplitude, but these desirable steps are not always possible.
Increasing the cutter speed is also often impractical because tool life will be substantially shortened in practice.
The use of helical-tooth end mills, similarly to helical gearing, is helpful in abating but not solving these problems.
The state of the prior art can be assessed from a review of relevant U.S. Patents.
In U.S. Pat. No. 4,285,618, Stanley, Jr. claims a milling cutter shown as an end mill provided with serrations on the cutting edges. The serrations are axially off-set in relation to a neighboring tooth. Whether or not such a cutter will reduce vibrations would need to be proved by tests.
In U.S. Pat. No. 4,963,059, Hiyama proposes an end mill wherein the flute helix angle is not the same for each flute. However, as the peripheral cutting edges are equally spaced around the cutter periphery in at least one position, the proposed design would provide only a partial solution. Also, a problem would arise during manufacture of said end mill as the metal available for formation of the tooth would vary significantly along the length of the cutter.
In U.S. Pat. No. 6,168,355, Wardell describes an end mill having a main body and ears extending outward from the point of the tool. Means for reducing vibration are not provided.
In U.S. Pat. No. 6,164,877, Kamata et al. disclose a formed shape cutter intended for cutting specially-shaped grooves. The relief angle of the tooth remains constant along the axial length of the tool. No anti-chatter means are seen.
Wardell, in a further U.S. Patent, discloses an end mill having a primary helical flute defining a low-angle cutting surface and a secondary flute for a high angle cutting surface. This arrangement will not solve the problems relating to tool vibrations.
A further end mill having a variable helix flute is seen in published U.S. Patent Application 2005/0105973 by MacArthur. The teeth of the cutter are equally spaced around the tool periphery.
From the prior art it is evident that no satisfactory solution is yet known.